linux-kernel - Re: [PATCH 09/14] mm, compaction: Ignore the fragmentation avoidance boost for isolation and compaction

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Date:   Tue, 18 Dec 2018 14:29:52 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Linux-MM <linux-mm@...ck.org>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>, ying.huang@...el.com,
        kirill@...temov.name, Andrew Morton <akpm@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/14] mm, compaction: Ignore the fragmentation avoidance
 boost for isolation and compaction

On Tue, Dec 18, 2018 at 02:58:33PM +0100, Vlastimil Babka wrote:
> On 12/18/18 2:51 PM, Mel Gorman wrote:
> > On Tue, Dec 18, 2018 at 01:36:42PM +0100, Vlastimil Babka wrote:
> >> On 12/15/18 12:03 AM, Mel Gorman wrote:
> >>> When pageblocks get fragmented, watermarks are artifically boosted to pages
> >>> are reclaimed to avoid further fragmentation events. However, compaction
> >>> is often either fragmentation-neutral or moving movable pages away from
> >>> unmovable/reclaimable pages. As the actual watermarks are preserved,
> >>> allow compaction to ignore the boost factor.
> >>
> >> Right, I should have realized that when reviewing the boost patch. I
> >> think it would be useful to do the same change in
> >> __compaction_suitable() as well. Compaction has its own "gap".
> >>
> > 
> > That gap is somewhat static though so I'm a bit more wary of it. However,
> 
> Well, watermark boost is dynamic, but based on allocations stealing from
> other migratetypes, not reflecting compaction chances of success.
> 

True.

> > the check in __isolate_free_page looks too agressive. We isolate in
> > units of COMPACT_CLUSTER_MAX yet the watermark check there is based on
> > the allocation request. That means for THP that we check if 512 pages
> > can be allocated when only somewhere between 1 and 32 is needed for that
> > compaction cycle to complete. Adjusting that might be more appropriate?
> 
> AFAIU the code in __isolate_free_page() reflects that if there's less
> than 512 free pages gap, we might form a high-order page for THP but
> won't be able to allocate it afterwards due to watermark.

Yeah but it used to be a lot more important when watermark checking for
high-orders was very different. Now, if the watermark is met for order-0
and a large enough free page is allocated, the allocation succeeds so
it's a lot less relevant than it used to be. kswapd will still run in
the background for order-0 if necessary so a heavy watermark check there
doesn't really help.

-- 
Mel Gorman
SUSE Labs